Histologic sections of the left main (LM) and other three major coronary arteries were studied in 152 patients. The lumen of the LM in 35 patients was greater than 75% narrowed; in thirty, 50-75%; and in 87, less than 50% narrowed. The patients with greater than 75% narrowing were younger. Angina pectoris and hyperlipoproteinemia, specifically type II, were more common (p less than 0.02) and acute transmural and healed subendocardial myocardial infarcts were less frequent (p less than 0.05) in the patients with greater than 75% LM narrowing than in those with less than 50% narrowing. Of the three other major coronary arteries, the average number narrowed in the patient with less than 50% LM narrowing, 2.4. Of the 35 patients with greater than 75% luminal narrowing of each of the other three major coronary arteries.